Pathology and disease states of the human eye lead to visual impairment and, in the worst case, loss of vision. Optical assessment of the eye's health is preferred because of the non-invasive nature of optical examination techniques. Common eye diseases include glaucoma, age-related macular degeneration, cataracts, retinal detachment, and diabetic retinopathy. Improved optical diagnostic techniques offer hope in quantifying disease progression and in tracking the effectiveness of disease treatments.
Previous work identifying depolarizing materials, alternatively called polarization scrambling materials, has largely focused on extracting information from the Mueller matrix. This work lies mainly in the field of polarimetry. It has been argued that, at least for optical coherence tomography (OCT), the Mueller calculus is not necessary. (See, S. Jiao and L. H. Wang, “Jones-matrix imaging of biological tissues with quadruple-channel optical coherence tomography,” J. Biomed. Opt. 7(3), 350-358 (2002).) Depolarization is a consequence of analysis of incoherent scattering. Because OCT detection is coherent, depolarization, or polarization scrambling, by biological tissue simply means that the tissue does not present a spatially consistent polarization response across independent neighboring detection cells. In other words, the polarization state of the scattered light varies from detection cell to detection cell, whenever the detection cells are separated by more than the diameter of a speckle cell. Thus, in coherent detection devices like OCT, the degree of polarization is meaningful only when examining clusters of detection cells spanning a number of speckle diameters.
Alternatively, depolarization is directly addressed within the Mueller calculus. While the Mueller calculus nominally describes incoherently detected light, conversion from a Jones matrix to a Mueller matrix is possible and well-known (See, for example, Appendix 4: Jones-Mueller Matrix Conversion of “Spectroscopic Ellipsometry” by Hiroyuki Fujiwara (2007). Coherent detection is described by a subset of Mueller matrices. The full Mueller matrix contains information on the intensity, retardance, diattenuation, and depolarization of a scattering material. Evaluating the Mueller matrix on a scatterer-by-scatterer basis provides this information for each scatterer. In general, however, it is impractical to resolve each scatterer. In a typical OCT system, the resolution of the illumination beam (the detection cell) is specified to be nearly the same size as a speckle cell. In this case, computing or averaging the Mueller matrix over multiple speckle diameters, where each detection cell covers a plurality of actual scatterers is generally more practical. Nominally, for Mueller matrix imaging, the Mueller matrix is obtained on a pixel-by-pixel basis for a given image size. The 4×4 Mueller matrix has 16 real elements, and complete resolution of the Mueller matrix implicitly resolves the depolarization elements of the matrix.
The Mueller matrix elements for a scattering tissue represent the relationship between the input and the output Stokes vectors through the equation: Ŝ=MS, where S is the Stokes vector representing the input beam, M is the Mueller matrix, and Ŝ is the Stokes vector representing the beam backscattered by the tissue. By illuminating the tissue with light of various known polarization states and computing the Stokes vectors of the backscattered light for each pixel of illuminated tissue, evaluation of the Mueller matrix for each pixel of illuminated tissue is possible.
The degree of polarization (DOP), , of light is the proportion of completely polarized light when the light is decomposed into a completely depolarized component and a completely polarized component. When light represented by Stokes vector, S, is decomposed into its completely polarized component, SP, and its completely depolarized component, SD, the DOP satisfies: S=(1−)SD+SP For Stokes vector
  S  =      (                                        S            0                                                            S            1                                                            S            2                                                            S            3                                )  the DOP satisfies =√{square root over (S12+S22+S32)}/S0.
The classical measure of the degree of depolarization imparted by a scattering medium is the depolarization index of the Mueller matrix M defined by Gil and Bernabeu:
            D      ⁡              (        M        )              =                  1                  3                    ⁢              1                  m          00                    ⁢                                    ∑                                          (                                  i                  ,                  j                                )                            ≠                              (                                  0                  ,                  0                                )                                              ⁢                                          ⁢                      m            ij            2                                ,where mij is the (i,j)th element of M.(See 20070146632 Chipman, Eq. 14). The depolarization index varies between zero and one. It is zero for the ideal depolarizer and one for non-depolarizing Mueller matrices. Once the Mueller matrix is known, the depolarization elements (the 9 Mueller matrix elements mij for i,j≧1) are known and a depolarization image can be constructed.
In “Segmentation of the retinal pigment epithelium by polarization sensitive optical coherence tomography,” Hitzenberger, et al., reported an alternate method for determining if tissue is depolarizing. Using a polarization sensitive OCT (PS-OCT) system with a polarizing beam splitter in the detection arm and two identical detection systems, they detected retardance data at each detection cell. Polarization preserving tissue returns consistent retardation values from neighboring scatters, while depolarizing tissue returns randomly varying retardation values from neighboring scatters. By computing statistics on retardation measurements in a neighborhood of a pixel, Hitzenberger determines that the tissue is depolarizing at any location where the variance of the retardation measurements exceeds a fixed threshold. In other words, the greater the variance in the retardation measurements, the greater the depolarizing nature of the scattering tissue.
Full resolution of the Mueller or Jones matrix is costly and/or time consuming. A typical PS-OCT system requires at least a polarizing beamsplitter and two detection channels to evaluate the polarization state of the return light (from which retardation and other polarization parameters can be derived) and depolarizing tissue can be located using statistics as shown by Hitzenberger. In this case, the cost is in additional hardware. Additionally, a PS-OCT system is relatively difficult to align and calibrate. Our invention resolves these problems by estimating the location of polarization scrambling tissue without resolving the Mueller or Jones matrix (i.e. without resolving the actual polarization state of the light) or adding additional hardware to the detection channels of a typical OCT system.